1. Field of the Invention
The field of the present invention is civil aeronautics and, in particular, noise generation from aircraft.
2. Description of the Related Art
Noise generated by civil airplanes, in particular upon takeoff, is a largely known nuisance and numerous innovations have been imagined to try and reduce it. One of the main sources of such a noise lies in the jet of the engines which are used at full power upon takeoff phase. Important works have been obviously conducted to try and reduce noise from jet engines, such as for example sawtooth-shaped herringbones for the ejection nozzle, either for the hot gas one, coming from the primary flow of the jet engine, or for the nozzle of the so-called cold gas which are coming from the secondary flow of the engine.
In order to limit the nuisances to which people living near the airports are submitted strict standards have been imposed, limiting the noise that can be perceived in various points located around the airplane, at different distances and in several directions with respect to the runway.
One of the particularly critical points to be respected by the designers of airplanes in terms of allowable maximal noise to obtain the certification for an airplane is located in a lateral position with respect to the airplane, at a distance of 450 m of the takeoff runway. The presence of the pylon, that is to say the mast supporting the engine by connecting it to the wing, locally generates at the level of the gas ejection, high turbulence levels in the flow, with as a consequence a very significant increase of the lateral noise of the engine. Such phenomenon is particularly acute for configurations where the pylon is prominent beyond the gas ejection plane, this becoming a very frequent configuration on the recent civil airplanes.
The results of numerical calculations or measurements implemented on a mock up in a wind tunnel, show quite well that the effects of interactions between the flow circulating along the pylon and the pylon itself generate a strong increase of the turbulence levels and, consequently, of the noise level. An important modification of the jet angular development radially around the pylon can also be noticed, thereby tending to orient the engine jet around the pylon along the direction of the wing.
Furthermore, the experience acquired shows that the introduction of the pylon, besides its influence on the increase of the sound level on a conventional ejection configuration, can also reduce considerably the efficiency of other devices being arranged to reduce the exhaust gas noise, such as herringbones or mixers arranged on the nozzles.
The presence of the pylon introduces in such a way, in terms of acoustics, an increase of the ejection noise on the lateral point of certification, which can vary between 2 to 3.5 EPNdB (Effective Perceived Noise in Decibels) according to the motive cycle, the pylon size and the ejection geometries being considered. The turbulences form small swirling structures which enlarge and radiate outside the turbo engine at low frequency and generate noise.
The need to reduce the jet noise being a constant preoccupation of the motorists, anybody sees quite well the interest to reduce noise at the source, i.e. by acting on the local turbulent flows around the pylon. The noise reduction potential seems to be even, finally, more important that the one brought by the implementation of herringbones of micro-jets on the periphery of the nozzle.